1. Field of the Invention
The present invention relates to a navigation system for vehicles such as automobiles, and more particularly to an information processing system and method which are well suited to estimate the position of a vehicle at high precision by the use of map data.
2. Description of the Prior Art
As stated in, for example, "Nikkei Electronics" dated Nov. 16, 1987, pp. 119-130, a prior-art on-board navigation system for a land surface vehicle such as automobile employs a method wherein the position of the vehicle itself, which is estimated by the use of a travel distance detected by a car speed sensor mounted on the vehicle, as well as a current azimuth angle obtained from a steering angle detected by a steering angle sensor or an attitude angle detected by a magnetic sensor similarly mounted, is displayed in superposition on map data.
There is also a method wherein, in order to heighten the estimation accuracy of the position of the vehicle itself, a receiver in a GPS (Global Positioning System) or a receiver for location beacons (sign posts), which are radio beacons installed on roads for transmitting the absolute positional information items thereof, is mounted, and the received information of the system or beacon is used in combination with the data of the aforementioned travel distance or current azimuth angle.
Further, there has been known a method wherein, in order to prevent an estimated current position from missing a road on a map because of an error, an estimative current position to be displayed is corrected onto the road by the use of the positional information of the road of map data. An example of the method is discussed in the official gazette of Japanese Patent Application Laid-open No. 56910/1986.
The first prior-art technique mentioned above is such that, on the basis of the initial position of the vehicle at the start of the travel of the vehicle or at the start of the display of the position of the vehicle, the travel distance and the current azimuth angle detected every moment are integrated to evaluate the current position at each of later points of time. Therefore, it has the disadvantage that the errors of the initial position, travel distance and current azimuth angle diverge accumulatively.
Since the second prior-art technique can directly estimate the position of the vehicle by the use of the GPS or the location beacons, a positional error does not diverge with time. Nevertheless, an error of several tens of meters to several hundred meters remains.
With the two prior-art techniques, accordingly, there has been the problem that, when the estimated current position is displayed in superposition on the map, the displayed position misses the road in spite of the traveling of the vehicle on the road.
In the third prior-art technique, the probability density of the current position at every moment is computed, and it is compared with a road position in the map data. When a place on the road whose probability exceed a certain fixed threshold value has been detected, it is displayed as being the current position. Herein, the probability density is assumed to be of a Gaussian distribution, and it is approximated with a small number of parameters. This method has had the problem that actually the probability density falls into a shape different from the Gaussian distribution on account of speed regulation, diversion, etc., so the estimation does not become optimal. Another problem is that, in someways of selecting the threshold value, the current position is forcibly displayed on the road in a case where the vehicle actually misses the road on which the current position is displayed and where it lies at a position, such as a back street, which is not contained in the road information of the map data.
Meanwhile, the positional accuracy of a vehicle has been enhanced by the following method: Road map data recorded on a CD-ROM is displayed on the CRT of a dashboard, and the start point of the vehicle is input on the map of the CRT by a cursor when the vehicle starts traveling. Then, the current position is displayed on the map of the CRT on the basis of a car speed and information on a traveling direction from a terrestrial magnetism sensor. In particular, an estimated trajectory is compared with a route pattern on the map, and the positional information and the map pattern are checked up at the intersection of roads or the bending point of a road, so as to estimate the position of the vehicle believed correct.
In this case, however, the errors of the azimuth and the car speed are corrected only at the travel of the vehicle through the featuring point and on each occasion. Accordingly, there has not been considered the problem that, once the vehicle has entered an erroneous route, the correction fails.
Further, the above method considers only the local correction on the error of the estimative current position attributed to the accumulation of the errors of the preceding azimuths and car speeds. As another problem, accordingly, there has been such a possibility that the estimated current position on the CRT will miss a road, or that the vehicle will enter a road not leading to a destination, while resorting to the estimated positional information which is different from the actual current position.